1. Field of the Invention
The present invention relates generally to light-sensitive positive photoresist compositions and particularly to compositions containing aqueous-alkaline soluble phenolic resin binders together with specific naphthoquinonediazide sensitizing agents having improved solubility, improved inhibition to dissolution, and high resolution capability.
Trends in circuit design are toward increasing the scale of integration and packing density, thereby increasing performance at a reduced fabrication cost per circuit element. The progress toward higher packing densities requires the feature size of the circuit elements to be correspondingly reduced, a trend which has been continuously accelerating and which has resulted in doubling the number of components per chip every year. In these scaled down very large integrated (VLSI) circuits the integration rate is higher than 100,000, wherein the design of 1.0 .mu.m rule or less is required. However, as this progress continues, the photolithographic processing used to delineate the circuit elements is rapidly approaching its resolution capability which is primarily limited by diffraction considerations. The resolution limit of an imaging system is a function of the wavelength of the exposing radiation, hence, higher resolution can be attained with shorter wavelength radiation. Realization of this limitation has resulted in an intensive and continuing development of other potentially higher resolution resist systems, exposure tools and associated processes.
2. Description of Prior Art
Most of the conventional positive type photoresists used for the production of integrated circuits consist of a matrix resin and a sensitizer based, in most cases, on a naphthoquinonediazidesulfonic acid ester of a substituted polyhydroxybenzophenone. The matrix resin is generally a cresylicformaldehyde novolac resin that is soluble in an aqueous alkaline solution and the naphthoquinonediazide esters function as a dissolution inhibitor for the matrix resin. Upon radiation exposure the sensitizer undergoes a structural transformation (known as the Wolff rearrangement), followed by reaction with ambient water to form a base-soluble indenecarboxylic acid that is no longer a dissolution inhibitor of the matrix resin in alkaline developer. As a result, the exposed region of the film is rendered more soluble then the unexposed, producing a differential solubility rate that is used to generate a positive-tone image of the mask.
In recent years, the positive type photoresist, based on a novolac matrix resin and a naphthoquinonediazidesulfonic acid ester sensitizer have become the material of choice in the microelectronic industry due, in part, to its potentially good resolution capabilities and dry etch resistance. In the case of VLSI, photoresists having high sensitivity and excellent stability during development and processing conditions are required to enable treatment of a great number of materials within a short period of time. However, in the prior art of positive acting photoresist compositions, attempts to minimize loss of residual film thickness in the unexposed areas of the film during development processes by increasing the amount of the naphthoquinonediazide sensitizer have produced lower sensitivity. On the other hand, when the amount of the naphthoquinonediazide sensitizer is reduced to enhance sensitivity, this results in an increase in the loss of residual film thickness in the unexposed areas of the film during the development making the product impractical. This is especially true in the case where the sensitizer used has a low diazo content, a large amount of the sensitizer is required in the photoresist composition to enhance resolution and unexposed film thickness retention. Incorporation of such large amounts of sensitizer at the expense of the novolac matrix resin, however, inevitably causes undesirable loss of adhesion of the resist film to the substrate, precipitation of the naphthoquinonediazide compound, or loss of resolving power of the photoresist.
Another fault of the prior novolac matrix resin-based positive type photoresists is the greatly reduced sensitivity in the mid-UV region (300 to 380 nm) of the ultraviolet spectrum relative to their performance in the near-UV region (380 to 450 nm). The reasons for this reduced performance are the results of several and cummulative undesirable optical properties. Firstly, the molar extinction coefficient of the naphthoquinonediazide sensitizers that are used in the prior novolac matrix resin-based positive type photoresist compositions are very high in the region of 300 to 380 nm compared to that at 436 nm. Secondly, the naphthoquinonediazide sensitizers undergo photochemistry upon ultraviolet radiation exposure which ultimately leads to a photoproduct that is transparent at 436 nm but strongly absorbs in the region of 300 to 390 nm, resulting in incomplete photobleaching. Finally, the novolac matrix resins used in most of the prior positive type photoresist compositions have a significant unbleachable absorbance in the region of 300 to 320 nm, but are essentially transparent above 350 nm.
For overcoming the above-mentioned problems, the present inventors have conducted extensive research to improve the useful properties of the prior art positive-type photoresists. As a result, it has been found that the above mentioned problems can be overcome by incorporating a specific naphthoquinone-(1,2)-diazide-(2)-sulfonyl compound to an alkali-soluble phenolic matrix resin.